1. Field of the Invention
The present invention relates to a nonreciprocal circuit device, such as an isolator or a circulator, used at a high-frequency band, such as a microwave band, and also relates to a communications device using the nonreciprocal circuit device.
2. Description of the Related Art
Recently, given the growing demand for miniaturized and inexpensive mobile communications apparatuses such as mobile telephones, there are similar demands for a miniaturized and inexpensive nonreciprocal circuit device. To meet this demand, the present assignee disclosed an isolator comprising a vertical capacitor, wherein single plate capacitors are used as capacitors for matching, and are provided vertical to the mounting surface of the isolator (Japanese Patent Application No. 9-252207, i.e. Unexamined Patent Publication No. 1999-97910).
This isolator comprises a permanent magnet provided on the inner face of a top yoke, a bottom yoke attached to the top yoke to form a magnetically closed circuit, a resin case provided on the lower face in the bottom yoke, a magnetic structure comprising three central conductors provided on a ferrite in the resin case, three capacitors for matching, and a termination resistor. Then, single plate capacitors comprising electrodes provided on both main faces of a dielectric substrate are used as the capacitors for matching in order to reduce costs, and each single plate capacitor C is provided vertical to the ferrite 55 to make the size of the isolator smaller, as shown in FIG. 6. FIG. 6 is a diagram showing the positional relationship, between a ferrite and a single plate capacitor. In the isolator mentioned above, the single plate capacitor C is provided at a position such that the bottom edge of the capacitor C is disposed less than half of the thickness of the ferrite 55 above the bottom face of the ferrite.
When the isolator is miniaturized, that is, when its constituent members are required to be miniaturized, a problem that the insertion loss increases is caused, and consequently it is demanded strongly to achieve both miniaturization and reduction of insertion loss.
When the above isolator is viewed from the side, the electrode face of the capacitor overlaps with the side face of the ferrite, whereby the electrodes of the capacitor obstruct the high-frequency magnetic field created by the central conductors, shortening the path of the magnetic field, and abruptly altering the high-frequency magnetic field around the central conductors. As a consequence, the magnetic components of the central conductors perpendicular to the DC magnetic field required for magnetic coupling are reduced, and insertion loss increases. That is, although the isolator described above can be more miniaturized by providing the capacitors vertically, it has a disadvantage that insertion loss cannot be reduced.
Furthermore, in the constitution described above, when the hot end electrode of the capacitor is provided to face the ferrite side, the ground terminal of the central conductors and the hot end electrode of the capacitor are liable to be short-circuited. Consequently, the distance between the capacitor and the central conductors is required to be increased, or an insulator is required to be disposed between the capacitor and the central conductors. As a result, this makes it more difficult to achieve miniaturization and low costs.
Accordingly, it is an object of the present invention to provide a nonreciprocal circuit device which can be inexpensively miniaturized by vertically providing a plate-like capacitor, and a communications device using the same.
In order to achieve the above objects, according to a first aspect of the present invention a nonreciprocal circuit device comprises a plurality of central conductors provided on a plate-like ferrite, to which a permanent magnet applies a DC magnetic field, and plurality of plate-like capacitors are connected to respective ports of the central conductors. The ferrite is provided so that both main faces thereof are parallel to the mounting surface, and the capacitors are provided such that electrode surfaces of the capacitors are substantially perpendicular to the mounting surface and respective bottom faces of the capacitors are disposed more than half of the thickness of the ferrite above the bottom face of the ferrite.
Preferably, the capacitors are disposed higher than the top face of the ferrite.
In another aspect of the present invention, the capacitors are single plate capacitors, comprising electrodes provided on both main faces of a dielectric substrate.
Alternatively, the capacitors are multilayer capacitors, comprising a plurality of dielectrics and a plurality of electrodes which are laminated alternately.
Furthermore, a communications device according to the present invention comprises the above nonreciprocal circuit device.
The present inventors have discovered that, in a nonreciprocal circuit device wherein the electrode faces of capacitors are disposed substantially perpendicular to the main face of a ferrite on which a plurality of central conductors are provided, the insertion loss of the nonreciprocal circuit device can be improved by changing the positional relationship between the ferrite and the capacitors.
That is, as will be explained clearly in the following preferred embodiment, the nonreciprocal circuit device can be miniaturized by providing plate-like capacitors substantially perpendicular to the ferrite. In addition, when the bottom faces of the capacitors are disposed above the bottom half of the thickness of the ferrite, it is possible to improve a level of insertion loss in practical use.
Furthermore, the insertion loss can be minimized by disposing the bottom face of the capacitors higher than the top main face of the ferrite.
Moreover, when a capacitor having electrodes on both main faces thereof, such as a single plate capacitor, is disposed at a position higher than the top main face of the ferrite, since the capacitor is not located on the side face of the ferrite, short-circuiting between the ground side of the central conductors and the hot end electrode of the capacitor can be prevented.
Furthermore other, a single plate capacitor or a multilayer capacitor can be used for the capacitors. A single plate capacitor is easily manufactured, enabling costs to be reduced. Furthermore, by using multilayer capacitors, the device can be further miniaturized.
Furthermore, the communications device according to the present invention comprises a single plate capacitor having characteristics described above, and therefore, it has excellent characteristics, being miniaturized and inexpensive.
Other features and advantages of the invention will be appreciated from the following description of embodiments thereof.